biostudies-literatureUnknownSanguankiattichai NEuropean Research CouncilWellcome TrustBiotechnology and Biological Sciences Research CouncilThe extracellular space (apoplast) of plants is an important molecular battleground during infection by many pathogens. We previously found that a plant-secreted β-galactosidase BGAL1 acts in immunity by facilitating the release of immunogenic peptides from bacterial flagellin and that <i>Pseudomonas syringae</i> suppresses this enzyme by producing a small molecule inhibitor called galactosyrin. Here, we elucidated the structure and biosynthesis of galactosyrin and uncovered its multifunctional roles during infection. Structural elucidation by cryo-EM and chemical synthesis revealed that galactosyrin is an iminosugar featuring a unique geminal diol attached to the pyrrolidine moiety that mimics galactose binding to the β-galactosidase active site. Galactosyrin biosynthesis branches off frobioRxiv : the preprint server for biology2025.02.13.638044https://www.ebi.ac.uk/biostudies/studies/S-EPMC11844564biostudies-literatureBacterial pathogen deploys iminosugar galactosyrin to manipulate plant glycobiology.PMC11844564203141/Z/16/ZBB/R017913/1206422/Z/17/Z101019324BB/T015128/1Drapal MSheng YTabak WWAChandrasekar BKrahn DYamamoto SFleet Gvan der Hoorn RALZhang PPreston GMHardenbrook NFraser PKato ASanguankiattichai NNash RBuscaill PKaiser MfalseBacterial pathogen deploys iminosugar galactosyrin to manipulate plant glycobiology.The extracellular space (apoplast) of plants is an important molecular battleground during infection by many pathogens. We previously found that a plant-secreted β-galactosidase BGAL1 acts in immunity by facilitating the release of immunogenic peptides from bacterial flagellin and that <i>Pseudomonas syringae</i> suppresses this enzyme by producing a small molecule inhibitor called galactosyrin. Here, we elucidated the structure and biosynthesis of galactosyrin and uncovered its multifunctional roles during infection. Structural elucidation by cryo-EM and chemical synthesis revealed that galactosyrin is an iminosugar featuring a unique geminal diol attached to the pyrrolidine moiety that mimics galactose binding to the β-galactosidase active site. Galactosyrin biosynthesis branches off fro2025-01-01T00:00:00Z2025 Feb2026-05-26T14:03:23.78Z2025-04-04T13:11:01.876ZS-EPMC118445643999030810.1101/2025.02.13.638044